Supercapacitors are electrical energy storage systems particularly advantageous for applications requiring electrical energy to be carried at high power. Their capability for charging and discharging rapidly, and their increased lifetime with respect to a high-power battery make them promising candidates for a number of applications. Supercapacitors generally consist of the association of two conducting porous electrodes with a high specific surface area, immersed in an ionic electrolyte and separated by an insulating membrane, known as a “separator”, which enables the ionic conductivity and avoids the electrical contact between the electrodes. Each electrode is in contact with a metal collector allowing the exchange of the electrical current with an external system. Under the influence of a potential difference applied between the two electrodes, the ions present within the electrolyte are attracted by the electrode surface having an opposite charge, thus forming an electrochemical double-layer at the interface of each electrode. The electrical energy is thus stored electrostatically by separation of the charges. The expression for the capacitance C of a supercapacitor is identical to that of a conventional capacitor, i.e.:
C=ε·S/e, where ε denotes the permittivity of the medium, S the surface area occupied by the double-layer, and e the thickness of the double-layer.
The capacitances achievable within supercapacitors are nevertheless much higher than those commonly reached by conventional capacitors, owing to the use of these electrodes with a maximized specific surface area, usually carbon-based, and of the extreme fineness of the electrochemical double-layer (typically of a few nanometers in thickness). These carbon-based electrodes must necessarily be conductive in order to ensure the transport of the electrical charges, porous in order to ensure the transport of the ionic charges and the formation of the electrical double-layer over a large surface area, and chemically inert so as to avoid any spurious reaction that might consume energy.
The main design parameters for supercapacitive devices are the total stored energy, the maximum deliverable power and the mass and volume densities of the two preceding quantities (i.e. the energy density and the power density). A compromise must in general be reached between these parameters, depending on the target applications. Some applications such as static mass storage devices require, above all, a large storage capacity without the maximum power being a truly limiting factor, whereas other applications, such as railroad equipment, require a high energy and power without the constraints on weight and on volume really being limiting, it being noted that the applications to automobile or aeronautical vehicles require a compromise in energy/power with very tight constraints on weight and on volume.
The energy E stored within a supercapacitor is defined according to the conventional expression for capacitors, i.e.:
E=½·C·V2, where V is the potential of the supercapacitor.
The capacitance and the potential are two essential parameters which need to be optimized in order to promote high performance in energy, the potential mainly depending on the nature of the electrolyte and notably on its electrochemical stability (it being noted that the main families of electrolytes are organic electrolytes, comprising an organic salt dispersed within an organic solvent, and aqueous electrolytes), and the capacitance depending of the porous texture accessible in practice by this electrolyte.
A unit supercapacitor cell (“unit cell” is understood in a known manner in the present description to mean an elementary cell of minimum size that is necessary for the autonomous operation of a supercapacitor and which comprises by definition two carbon-based porous electrodes separated by an insulating membrane and impregnated with an electrolyte) usually comprises two current collectors allowing the current to be carried as far as the two electrodes. In order to be able to reach a high-power operation, the resistance to the passage of the current within the unit cell must be very small, owing to the fact that this resistance leads to losses by Joule heating which decreases the efficiency of the supercapacitor. This resistance is the sum of the resistances of the various components of the unit cell, notably the resistance of the electrolyte and that of the current collectors. In order to limit the contribution of the resistances of the collectors, it is necessary to use for the latter metals with a high conductivity which must furthermore be inexpensive and able to be easily fashioned, notably such as copper and aluminum. However, in the case of an aqueous electrolyte, the use of these metals poses problems of chemical and electrochemical stability owing to the fact that they will corrode at the oxidation potentials typically used in an aqueous solution.
A supercapacitor cell (“cell” is understood in a known manner in the present description to mean a physical unit composed of one or more unit cells connected together) is not only composed of at least two carbon-based electrodes separated by a membrane and of two collectors, but furthermore of a leak-tight packaging that is impermeable to gases and to liquids and of components ensuring the leak-tight and impermeable closure of the packaging. However, as only the active material of the electrodes contributes to the energy storage function of the cell, the weight and the volume of its other components, including the packaging, must be minimized without however limiting the performance of the cell.
Generally speaking, the design parameters of a supercapacitive module are expressed in terms of operating voltage and overall capacitance depending on the target application and on the environment of use of the module by linking the cells in series or in parallel. Linking in series allows the unitary voltages across the terminals of the cells to be summed, but to the detriment of the overall capacitance whose inverse is equal to the sum of the inverses of the unitary capacitances. In contrast to this, linking cells in parallel allows the capacitances of the cells to be summed but without modifying the voltage across the terminals of each cell.
In this context, the document U.S. Pat. No. 6,998,190 B2 may be mentioned which teaches the use in a supercapacitor of a stack of unit cells enveloped in two packaging plastic films at the junction of which current collectors are bonded or fused.